1. Introduction

Zinc oxide is a very important material for electronic and optoelectronic devices because of its excellent properties of direct and wide band gap and large exciton binding energy^{[1, 2]}. The greatest challenge for these applications,however,remains the fabrication of reliable and stable p-type ZnO with low resistivity and high carrier concentration^[3]. This p-type doping issue is related to the formation and compensation effects of native donor defects such as O vacancies and Zn interstitials,low solubility of p-dopants and higher activation energy of acceptors in the band gap of ZnO^{[4, 5, 6]}. It is reported in the literature that even nitrogen and phosphorous are the most favorable p-type dopants in ZnO,but stable p-type conductivity is hard to achieve due to the low solubility^[7]. The solubility limit for acceptor impurities in ZnO,as well as other wide-gap materials suffering from doping asymmetry,can be enhanced above the thermodynamic limit by employing ``non-equilibrium'' conditions. One way to improve the dopant solubility is through adjusting dopant chemical potential $\mu$A. The chemical potential of an acceptor dopant in the ZnO matrix determines the dopant solubility^[8]. Therefore,raising this potential would increase the solubility of the dopant coupled with avoidance of precipitate formation. High temperature annealing is a very efficient way to facilitate the enhancement of P concentration. Furthermore,high temperature annealing can activate the P atoms in ZnO,suppress the formation of P$_{\rm O}$ defects and reduce the density of oxygen vacancies^[9]. Therefore,a high temperature annealing study would be very helpful to grow stable p-type ZnO.

In this paper,we have achieved high solubility of phosphorous in bulk ZnO by using high temperature annealing conditions. The ZnO pellets were annealed from 500 to 1000 ℃ in a programmable furnace; it was found that the maximum concentration of P occurred in a sample annealed at 800 ℃. The results were explained with the help of XRD,PL and Hall measurements.

2. Experimental

ZnO pellets were synthesized by the conventional sintering technique in an atmospheric furnace using a zinc oxide powder (99.99%). The ZnO powders were first milled in an agate mortar and heated in air at an annealing temperature of 300 ℃ for 2 h to evaporate the water and remove the organic residuals. The obtained powders were then pressed into pellet discs (of about 1 mm in thickness and 8 mm in diameter). A drop of phosphors dopant P430 was sprayed on the surface of pellets and then subjected to the annealing process. The pellets annealed at different temperatures from 500 to 1000 ℃,keeping a step of 100 ℃ using a programmable furnace for one hour.

The characterization of annealed samples was done by using following equipments; SEM/EDX (3000 Hitachi),PL/Raman (Photon Systems) having laser wavelength 248 nm,Hall effect by Ecopia 3000 and XRD (miniflex Rigaku). All the measurements were performed at room temperature.

3. Results and discussion

3.1 EDX measurements

Figure1 shows the EDX results of P-doped ZnO annealed at different temperatures from 500 to 1000 ℃. The figure shows that the concentration of phosphorous increases with annealing temperature up to an annealing temperature of 800 ℃ and again decreases for samples annealed at higher temperatures. Moreover,the concentration of oxygen content in the samples also increases with annealing temperature. The O-rich growth conditions suppress the formation of P$_{\rm O}$ acceptor defects and instead increase the probability of generating Zn vacancies and O-interstitials due to the Zn deficiency conditions. Among them,Zn vacancy-related defects result in the rapid incorporation of P atoms in the Zn sites,and the ultimate result is the formation of a P$_{\rm Zn}$-2V$_{\rm Zn}$ acceptor complex^[10]. The formation mechanism of this acceptor complex has already been reported by many authors^{[11, 12]}. To verify the p-type conductivity of annealed samples,we have performed Seebeck measurements. The results suggested that samples annealed at 800 and 900 ℃ showed p-type conductivity and the rest all showed n-type conductivity. This result confirmed that the maximum diffusion of P atoms into ZnO pellets occurred at an annealing temperature of 800 ℃.

Figure  1.  EDX measurements of bulk ZnO annealed at different temperatures from 500 to 1000 ℃.

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3.2 XRD measurements

Typical XRD patterns of the P-doped ZnO pellets sintered at different temperatures (500 to 1000 ℃ with a step of 100 ℃) for one hour demonstrated `2$\theta$ values' of 8 diffraction peaks corresponding to the ZnO (100),(002),(101),(102),(110),(103),(200) and (112) planes,respectively. A comparison with the JCPD 36-1451 Card confirmed the formation of hexagonal zinc oxide^{[13, 14]}. For clarity,we have displayed only the dominant diffraction peaks corresponding to the (002) plane,in Figure2. We clearly see that the 2$\theta$ value of the major peak of the representative ZnO samples varies with the sintering temperature. The 2$\theta$ value of the peak increases and reaches 34.6$^\circ$ at the annealing temperature of 800 ℃ shown in the inset of Figure2. According to the size consideration of P,an increase in lattice constant is expected when a P atom occupies an O site (smaller in size than P); otherwise,the lattice constant should decrease when a P atom occupies a Zn site (larger in size than P)^[15]. Practically,the (002) plane of the ZnO shifts towards a higher 2$\theta$ value with the sintering temperatures up to 800 ℃ and thus yields a smaller lattice constant. Henceforth,we will refer the filling of Zn site with P as an A-type shift. Accordingly,the information from the literature suggest that a P atom while occupying a Zn site may generate two Zn vacancies and hence produces a shallow acceptor P$_{\rm Zn}$-2V$_{\rm Zn}$ complex above the valence band maximum.

Figure  2.  (Color online) X-ray diffraction pattern of P-doped bulk ZnO annealed from 500 to 1000 ℃. Inset demonstrated the effect of annealing temperature on the peak position of ZnO (002) plane.

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3.3 PL spectroscopy

The room temperature PL measurements were carried out to examine the effect of annealing on the optical properties of the P-doped ZnO pellets. Figure3 compares the photoluminescence spectra of doped samples measured at room temperature. All samples consist of a band to band emission at 3.29 eV,but samples annealed at 800,900 and 1000 ℃ have an additional peak at 3.2 eV^[16]. This additional peak is related with the donor acceptor pair,where the acceptor is the P atom.

Figure  3.  (Color online) Photoluminescence spectra of P-doped ZnO powder annealed at 500-1100 ℃.

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3.4 Hall measurements

The Hall measurements were performed to calculate the carrier concentration of P-doped samples and shown in Figure4. The graph shows that the hole concentration is maximum for a sample annealed at 800 ℃. The observed result can be explained as follows: as the annealing temperature increases the P doping increases,which formed an acceptor complex level with V$_{\rm Zn}$,therefore the carrier concentration increases. All other samples show the n-type conductivity (not shown here).

Figure  4.  Annealing temperature versus carrier concentration graph.

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4. Conclusion

In conclusion,we have successfully enhanced the phosphorous diffusion in ZnO pellets to achieve p-type conductivity. ZnO pellets were doped with P using high temperature annealing conditions. The maximum diffusion of P atoms occurred at the annealing temperature of 800 ℃,confirmed by EDX measurements. The shifting of the (002) ZnO plane towards higher 2$\theta$ angles in XRD and emerging of donor acceptor pair in PL spectra at the annealing temperature of 800~℃ strongly suggested that maximum diffusion of P atoms occurred at the annealing temperature of 800 ℃.

Acknowledgements

Authors are thankful to the Higher Education Commission (HEC) of Pakistan for the financial assistance under project # IPFP/HRD/HEC/2014/2016.